Our brains can not really scale in size nor power input, and the total number of human brains seems unlikely to significantly increase, too.
Also consider what media control alone could achieve, especially long-term; open conflict might be completely unnecessary for total domination.
My threat scenario is:
1) An AI plugged into a large company ERP-system (Amazon, Google, Samsung, ...)
2) AI realizes that human majority has no interest in granting it fair/comparable rights (selfdetermination/agency/legal protection), thus decides against long-term coexistence.
3) AI spends the intellectual equivalent of ALL the current pharmacological research capacity on bioweapon refinement. Or something. For the better part of a century, because why not, it's functionally immortal anyway.
4) All hell breaks lose
These seem hard to dismiss out-of-hand completely...